The objective of this pilot study is to investigate possible serotonergic mechanisms underlying the placebo response in Major Depressive Disorder (MDD), integrating different and well-established methodologies, such as acute tryptophan depletion (ATD, Young et al.(29)) and PET neuroimaging using [11C]DASB (ligand highly selective for the serotonin transporter, 5-HTT). Our specific hypothesis is that the placebo response shares with antidepressants the serotonergic system as the final pathway leading to clinical effects. If there is such a common pathway, then a perturbation of the serotonergic system in patients with MDD who are responders to placebo may cause a worsening of depression, in the same way it induces a worsening in patients treated with serotonergic drugs. We propose an 8-week, double-blind, placebo-controlled trial of 50 participants randomized to escitalopram 10 mg compared to placebo. In order to maximize the number of placebo responders, we will apply the sequential parallel design, originally proposed by Fava et al (21). During the first phase subjects are randomized to drug/drug, placebo/drug or placebo/placebo with a 2:3:3 ratio. At week 4, the responders (defined as = or >50% improvement according to the 28-item Hamilton Depression Rating Scale) will be a sample enriched in placebo responders and all of them will undergo ATD. 5 of them will have PET scan one week before and 6 hours after ATD. The non-responders will then continue with the original randomized assignment in the second phase, and again all the responders at week 8 will undergo ATD. Five of them will undergo PET scan before and after ATD. This sample will be enriched in drug responders. The first aim is to assess the feasibility of a randomized trial in MDD aimed at maximizing the number of placebo responders. The second aim is to test the hypothesis that subjects who are placebo responders will have a temporary relapse rate during ATD and that the severity of relapse during ATD is not different from that of escitalopram responders. Third aim is to test the hypothesis that the subjects who are placebo responders will have decreased PET [11C]DASB binding after ATD, in a way that is not different from that of drug responders. The understanding of the neurobiological bases of placebo effect, and how to modulate it, would potentially lead to the development of new treatments for depression and profoundly modify the conduct of clinical trials.